In line with the recent rapid spreading of mobile phones adopting a communication system that allows simultaneous transmission and reception, such as CDMA and W-CDMA, demands for antenna sharers are increasing. Technology for configuring antenna sharers mainly employs SAW (Surface Acoustic Wave) element, Boundary Elastic Wave element, or BAW (Bulk Acoustic Wave) element, taking into account their small and short size and advantage for mass production.
In general, an antenna sharer have two filters (transmission filter and reception filter) for separating signals in transmission band and signals in reception band, which is a higher band adjacent to the transmission band. In particular, a ladder filter, in which series resonators and parallel resonators are connected in a ladder structure, is adopted as the transmission filter.
For example, Band 8 specified by 3GPP (3RD Generation Partnership Project) has a distance (crossband) of 10 MHz between the transmission band and reception band (1.09% if expressed by fractional bandwidth). This is extremely narrow compared to Band 5 that is often used in conventional antenna sharers. The distance between the transmission band and reception band in Band 5 is 20 MHz (fractional bandwidth: 2.33%). One known technology for securing sharpness corresponding to this narrow crossband is to weight IDT (Inter Digital Transducer) of the resonator in the transmission filter.
A prior art related to the present invention is, for example, PTL 1. However, PTL 1 aims to achieve narrow band, and thus no method for realizing an antenna sharer for wide band such as Band 8, whose band width is 35 MHz (fractional bandwidth: 3.9%), is disclosed.
In addition to Band 8, Band 3 is also a wide band, and thus antenna sharers that satisfy both low loss and sharp attenuation characteristic will be needed.
However, the known prior art fails to achieve sufficient sharpness if low loss is also achieved in the wide band. In addition, in a transmission filter of the conventional antenna sharer in the above PTL 1, if sharpness is increased to secure sufficient attenuation in the reception band, the transmission band width is narrowed. On the contrary, this increases losses in the wide transmission passband. In particular, in the transmission and reception passband of the above band 8, a fractional bandwidth is as wide as 3.9%. Accordingly, it is extremely difficult to retain a small loss over the wide transmission passband.